Chemical reactions in which, a single substance splits up into two or more simpler substances are called decomposition reactions. These reactions are carried out by energy, supplied by different sources. The required energy can be supplied by heat (thermolysis), electricity (electrolysis), or light (photolysis). Let’s talk about photolysis reactions (not photosynthesis): Photolysis (also called photodissociation and photodecomposition) is a chemical reaction, in which a chemical (an inorganic or an organic) is broken down by photons and is the interaction of one or more photons with one target molecule. The photolysis reaction is not limited to the effects of visible light, but any photon with sufficient energy (higher than the dissociation energy of the targeted bond) can cause the chemical transformation of the said (inorganic or organic) bond(s) of a chemical. Since the energy of a photon is inversely proportional to the wavelength, electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X-rays, and γ -rays, can also initiate photolysis reactions. Like all other peroxides, hydrogen peroxide (H2O2) also consists of a relatively weaker O−O bond, which is susceptible to light or heat. The net equation for the reaction is: 2H2O2⟶2H2O+O2 The step-wise reaction mechanism is suggested as follows (Ref.1): H2O2+hν⟶2HO∙ HO∙+H2O2⟶HO−O∙+H2O HO−O∙+H2O2⟶2HO∙+H2O+O2 Using isotope studies it was confirmed that the O2 formed is cleanly derived from H2O2 (Ref.2). Notes: The rate increases rapidly in the presence of catalysts such as MnO2 and KI (Ref.2). The rate of decomposition is slow at room temperature, but it increases with temperature. It is believed to be due to thermal decomposition of H2O2, which seemingly accelerates the photolysis (Ref.3). References: 1.J. P. Hunt, H. Taube, “The Photochemical Decomposition of Hydrogen Peroxide. Quantum Yields, Tracer and Fractionation Effects,” J. Am. Chem. Soc. 1952, 74(23), 5999–6002 (https://doi.org/10.1021/ja01143a052). 2.A. E. Cahill, H. Taube, “The Use of Heavy Oxygen in the Study of Reactions of Hydrogen Peroxide,” J. Am. Chem. Soc. 1952, 74(9), 2312–2318 (https://doi.org/10.1021/ja01129a042). 3.F. O. Rice, M. L. Kilpatrick, “The Photochemical Decomposition of Hydrogen Peroxide Solutions,” J. Phys. Chem. 1927, 31(10), 1507–1510 (https://doi.org/10.1021/j150280a004). Interestingly, as early as 1934 Haber and Weiss (Ref. 1) proposed that HO2∙ is formed in the decomposition of hydrogen peroxide. The subsequent reaction of the transient superoxide radical anion with hydrogen peroxide has been determined to also form the hydroxyl radical (Ref. 2). The higher pH version of the last reaction is, therefore, best represented as: O2∙−+H2O2⟶2HO∙+OH−+O2 And as H++OH−=H2O, the net product formation is not altered. However, alkaline H2O2 is well known to be less stable than acidic hydrogen peroxide (Ref.3) even in absence of light exposure, which accelerates its decomposition (which can involve radical pathways) liberating oxygen! References: 1.Haber Fritz and Weiss Joseph, 1934, The catalytic decomposition of hydrogen peroxide by iron salts. Proc. R. Soc. Lond. A 147: 332–351 http://doi.org/10.1098/rspa.1934.0221 2.TOSHIHIKO OZAWA, AKIRA HANAKI, Reactions of Superoxide with Water and with Hydrogen Peroxide, Chemical and Pharmaceutical Bulletin, 1981, Volume 29, Issue 4, Pages 926-928. https://doi.org/10.1248/cpb.29.926 3.https://www.researchgate.net/figure/Effect-of-pH-on-the-decomposition-of-hydrogen-peroxide-H-2-O-2-0-800-mg-l_fig1_234110563, from Yazıcı, Ersin & Deveci, Haci. (2010). Factors Affecting Decomposition of Hydrogen Peroxide. https://doi.org/10.13140/RG.2.1.1530.0648.